When adding -Wextra we also add -Wno-ununsed-parameters since that
function means well enough, but is really annoying in practice.
So the warnings we fix here are basically all comparsions between
signed and unsigned values.
We were previously just doing fprintf;exit at each point, but I
wanted to add file and line-number details to all messages, so it
makes sense to use a single macro for that.
It's even enough to check if a "missing" header was accidentally
left off the list in the call to restrict_headers. (And it's
cheap since we only check in case no such header was found in the
message.)
This is in preparation for a new, public notmuch_message_t.
Eventually, the public notmuch_message_t is going to grow enough
features to need to be file-backed and will likely need everything
that's now in message-file.c. So we may fold these back into one
object/implementation in the future.
